1. Field of the Invention
The present invention relates to piezoelectric transformers, and more particularly, to a piezoelectric transformer for use in back-light inverters for liquid crystal displays, inverters for lighting fluorescent lamps or high-voltage power circuits for copiers.
2. Description of Related Art
One example of a conventional piezoelectric transformer is shown in FIGS. 13(A) and 13(B). This piezoelectric transformer is called a Rosen-type and includes a piezoelectric transforming element 5 having a rectangular piezoelectric plate 1 formed of piezoelectric ceramics on which input electrodes 2, 3 and an output electrode 4 are disposed. The input electrodes 2, 3 are located on two opposite major surfaces (top and bottom surfaces in FIGS. 13(A) and 13(B)) of the piezoelectric plate 1 and extend along one half portion of a length of the piezoelectric plate 1. An output electrode 4 is located on an end surface of the other half portion of the piezoelectric plate 1.
In this piezoelectric transformer, the piezoelectric transforming element 5 is vibrated in a second order mode (.lambda. mode) wherein, as shown in FIG. 13(B), vibration nodes (node points) N.sub.1, N.sub.2 at which the displacement of vibration is substantially zero occur at positions spaced from each other by a quarter length of the piezoelectric plate 1 from respective ends thereof so that vibration antinodes at which the vibration displacement has a maximum value occur at a center and at the two ends of the piezoelectric plate 1. In a piezoelectric transformer of the type shown in FIG. 13(A), therefore, the piezoelectric transforming element 5 is held by resilient members 6 formed of resin, rubber, or the like. The resilient members 6 are located at vibration node regions corresponding to the vibration nodes N.sub.1, N.sub.2 at which the vibration displacement has a minimum value such that the piezoelectric plate 1 is not hindered from vibrating a maximum amount.
The input electrodes 2, 3 and the output electrode 4 are respectively soldered with metal wires 7 so that the input electrodes 2, 3 and the output electrode 4 are respectively electrically connected to input-side and output-side terminal electrodes.
The inventors of the invention described and claimed in the present application have determined that the piezoelectric transformer having a structure as described above and as shown in FIGS. 13(A) and 13(B) experiences the following problems (1)-(5):
(1) Since the electrical connections of the input electrodes 2, 3 and the output electrode 4 to the input and output terminal electrodes are made via the metal wires 7, the metal wires 7 often cause a short-circuit or a wire-disconnection condition due to contact between one or more of the metal wires 7 with other electronic parts which have a large mounting area and which are located on a printed circuit board next to the transformer 1. PA1 (2) The soldering of the metal wire 7 to the input electrode 2, 3 and the output electrode 4 requires changing the orientation and direction of the piezoelectric transforming element 5 depending upon the location of the input electrodes 2, 3 and the output electrode 4, so that the soldering operation for connecting the metal wires 7 to the electrodes 2-4 is complicated and increases the process steps, thereby preventing improvements in mass productivity. PA1 (3) Since the piezoelectric transforming element 5 is mounted within resilient members 6 made of resin, rubber or the like at the node regions corresponding to the nodes N.sub.1, N.sub.2 of the piezoelectric plate 1 and the resilient members 6 are arranged so as to be located on both of the opposite major surfaces of the piezoelectric plate 3, it is difficult to make the component sufficiently thin and to manufacture it at relatively low cost. PA1 (4) The solder for electrically connecting the metal wire 7 to the input electrodes 2, 3 and the output electrode 4 hinders the piezoelectric plate 1 from vibrating, causing a decrease in the pressure-increase ratio and conversion efficiency for the piezoelectric transformer. PA1 (5) Vibration stresses are concentrated at soldered portions where the metal wires 7 are joined to the input electrodes 2, 3 so that, when vibration is great at such soldered portions, the metal wires 7 are apt to break in the vicinity of the connection to the input electrodes 2, 3 or the output electrode 4.